The present invention relates to the calibration and correction of images produced by a gamma camera and, in particular, to the correction of energy-dependent spatial distortion errors.
Gamma cameras are typically subject to three types of errors. The first of these errors is spatial non-linearity.
Non-linearity is caused by phenomena known as "coordinate bunching," whereby scintillation events are mistakenly positioned towards the center of the photomultiplier tubes (PMTs). At a particular .gamma.-ray energy this positional error has two main causes: the collection efficiency of the PMTs vary non-linearly based on relative position of the scintillation events to the photocathodes; and variation in sampling the scintillation events occurs because a discrete number of PMTs are used to report events that occur over a continuous crystal surface.
The second type of error is non-uniformity. The main source of non-uniformity is again spatial distortion errors, which lead to variation in comparative pixel sizes.
Referring to FIG. 1, an ideal pixel has dimensions a x a. If the pixel is illuminated by .gamma.-rays having flux N. Then the count density in the pixel equals EQU d=N/a.sup.2.
If the spatial distortion results in changing a to a.+-..DELTA.a (FIG. 2), then the resulting density is equal to: EQU d'=N/(a.+-..DELTA.a).sup.2 .about.d/(1.+-.2.DELTA.a/a)
Thus, 1% spatial distortion would result in approximately 2% density error, or, in a 2% spatial non-uniformity.
The third type error is energy-dependent error. Energy-dependent errors are primarily the result of differing penetration/absorption depths within the scintillation crystal for .gamma.-rays of different energies.
For a scintillator crystal of density .rho., the .gamma.-ray intensity I remaining after traversal of a thickness t is given by: EQU I=I.sub.0 exp(-t.rho./.lambda.),
where I.sub.0 is the .gamma.-ray intensity before the crystal, and .lambda. is the .gamma.-ray mean free path in the crystal and is energy dependent. Referring to FIG. 3, the function I/I.sub.0 is plotted for various .gamma.-ray (or photon) energies for a thickness of 9.5 mm.
This varying interaction depth results in different solid angle geometries with respect to the PMTs. This changes the effective acceptance angle for each tube. Therefore the coordinate bunching behavior of the detector varies with .gamma.-ray energy.
U.S. Pat. Nos. 4,424,446, 4,808,826 and 4,817,038 show known methods and apparatus for correcting gamma camera errors and are incorporated herein by reference as if fully set forth herein.
Referring to FIG. 4, a prior art gamma camera 10 is typically calibrated by illuminating the detector 12 of the camera with a uniform calibration source 14 of photons of a particular energy, for example, 140 KeV. A calibration mask 16 containing a precise grid of holes is placed between the detector and the source. An image of the mask is compared to the actual hole geometry and an array of position or linearity corrections corresponding to each point in the image is calculated. In these prior art cameras, the same corrections are used whether the photons of interest are at the calibration energy or at a much different energy. Actual energies of interest may range, for example, from 50 to 511 KEV.
Unfortunately, for the reasons discussed above, these corrections are not constant with respect to photon energy. This results in imaging errors for photons at energies different than that of the calibration photons.
Referring to FIG. 5, an image of a calibration mask is shown using a Tc99m source after calibration for a Tc99m source (140 KeV). FIG. 6 shows an image of the same mask produced by a Ga67 source (93, 184 and 296 KeV) using the Tc99m calibration. Distortion of the image is apparent.